In some instances, shaft cracks have developed in high-energy rotating machinery. Sometimes these cracks propagate and cause the shaft to completely sever and fail during operation. These shaft failures, although infrequent, usually occur at inopportune times. These failures cost the equipment owner substantial financial losses for repairs and from equipment downtime. These unexpected shaft failures can also cause secondary damage to surrounding equipment. And in some rare cases, shaft failures can injure plant personnel.
In the past, rudimentary shaft crack detection was possible by monitoring the lateral vibration of the rotating shaft. These lateral vibration measurements are most frequently taken at the exposed section of the shaft, between the driver (motor) and driven component (e.g., pump, turbine or generator). The shaft crack must be quite deep before the shaft lateral vibration characteristics change sufficiently to make detection possible. Typically, the shaft crack must be in excess of 50% of the shaft diameter for a solid shaft and nearly through wall for a hollow shaft for detection to be possible from lateral vibration measurements. However, if a crack has grown to the extent that it can be detected by monitoring the shaft's lateral vibration, then shaft failure is imminent. In some instances, the shaft has less than 24 hours of available operation prior to complete failure after detection by lateral vibration measurements.
Monitoring the torsional vibration characteristics of a rotating driver, shaft and driven component allows for earlier detection of a shaft crack. The standard method of monitoring shaft torsion is accomplished by mounting strain gages on the surface of the exposed shaft and transmitting this strain data from the shaft using slip rings. Many layers of slip rings are needed to make the needed electrical contacts between the torsion strain gage rosetta and the respective strain gage signal conditioning.
Recently, techniques have been developed to transmit the strain gage data to the signal conditioning using telemetry. For the telemetry device a battery or inductance coil, wrapped partially around a shaft, is needed to provide sensor electrical power. These strain gage devices, utilized to measure shaft torsion on-line, are expensive, difficult to use and lacking in torsion signal resolution for proper shaft crack detection.
Both of these methods entail significant alteration of the shaft and require open space on the shaft which is normally unavailable. Some torque meters require cutting of the shaft and insertion of the transducers. Electrical noise can be induced in the data transmission, leading to unacceptable errors. The insertion of mechanical sensing and data collection may itself reduce the useful life of the rotating part due to additional weight, misalignment and wear.
Therefore, a need exists for a method of detecting cracks in a rotating shaft using a torque sensor not mounted on the shaft. In addition, a need exists for a method of detecting and estimating the depth of a crack in a rotating shaft at an early stage prior to actual shaft failure.